isisd: avoid fast-reroute on down adjacency when the interface is down

isisd: avoid fast-reroute on down adjacency when the interface is down

When an IS-IS interface is coming down, fast-reroute may be triggered
twice: a first time after the detection of the interface down event and
a second time after the detection of the adjacency down (because of the
expiration of the ISIS Hello or BFD timers).

Avoid a BFD down event from running fast-reroute another time if the
interface was already detected down.

Signed-off-by: Louis Scalbert <louis.scalbert@6wind.com>